1. Field of the Invention
The present invention relates to a magnetic recording medium for use in a magnetic recording/reproducing device having large capacity and high recording density, and a method for producing such a magnetic recording medium.
2. Description of the Prior Art
There is a tendency in a magnetic recording/reproducing device toward high density recording in order to achieve large capacity with a compact size. In the field of a hard disk drive, which is a typical magnetic recording device, a device having a recording density per unit of area (surface recording density) more than 1 Gbit/in.sup.2 has been already commercially available. It is expected to see a technological advancement so rapid that a device with 10 Gbit/in.sup.2 will be commercially practical in a few years.
An improvement in medium performance and head-disk interface performance, and an enhancement in a linear recording density resulting from the advent of new signal processing techniques such as partial response have allowed such high density recording. Recently, however, a tendency toward higher track density is greater than a tendency toward higher linear recording density, and this constitutes a main factor for the improvement in the surface recording density. This tendency is caused by the fact that a magneto-resistive type head (MR head) having more excellent reproducing output performance than that of a conventional induction type magnetic head has been put into practical use. At present, the magneto-resistive head allows signals in a track width as small as several gm to be reproduced in a good SIN ratio. In the near future, the head performance will further improve, and a track pitch will be in a sub-micron region.
When an MR head is used to reproduce signals, the signals to be reproduced are more affected by noise stemming from a recording medium, as compared to a conventional induction type magnetic head. For this reason, it is more important to reduce medium noise. In order to reduce noise, it is effective to minimize crystal grains in a magnetic layer and to isolate magnetic grains.
Conventionally, it is known to perform sputtering, for example, a CoCrPt based magnetic layer at a substrate temperature as high as 300.degree. C. or more, in order to facilitate the isolation of magnetic grains. This method allows atoms sputtered onto a substrate to move easily on the substrate. As a result, a magnetic layer is formed comprising CoPt based magnetic crystal grains and Cr-rich non-magnetic substances at grain boundaries between the magnetic crystal grains. The formation of the non-magnetic substances at the grain boundaries facilitates the isolation of the magnetic crystal grains.
However, according to the method of heating the substrate so as to facilitate the movement of Cr to the grain boundaries, the atoms that have reached the substrate can move readily so that the magnetic crystal grains become large. Thus, it is difficult to make the crystal grains smaller. A large size of crystal grains is detrimental to a decrease in medium noise.